1. Field of the Invention
This invention relates to a magnetic recording medium for thermally assisted recording, and relates to an improvement to enhance the durability and reliability of such a magnetic recording medium.
2. Description of the Related Art
Thermally assisted recording is technology in which the surface of a magnetic recording medium is irradiated with light, and the heating resulting from absorption of light by the medium is used to lower the coercive force Hc of the magnetic recording layer, to assist writing by a magnetic head, enabling magnetic recording at higher recording densities.
A magnetic recording medium is manufactured by layered deposition, in order on a nonmagnetic substrate, of an underlayer, intermediate layer, magnetic recording layer, protective layer, and lubricant layer. Among these layers, it is the magnetic recording layer that holds magnetic data related to recording. In principle, if this magnetic recording layer is heated, the purpose of thermal assist can be attained; but if possible, it is desirable that only the magnetic recording layer be heated, without heating the other layers.
However, heating of the magnetic recording layer in an actual magnetic recording medium is performed by causing light generated outside the magnetic recording medium to be incident on the uppermost surface of the magnetic recording medium. As a result, because each of the above-described layers which are layered on the surface of the nonmagnetic substrate of the magnetic recording medium is extremely thin, normally from several nanometers to several tens of nanometers in thickness, a portion of the incident light reaches not only the magnetic layer (magnetic recording layer), but also to the depth of the intermediate layer and underlayer which are layers below the magnetic recording layer. The light thus arriving is absorbed by the intermediate layer and underlayer, and is converted into thermal energy, heating the intermediate layer and underlayer.
Further, because the magnetic recording layer comprises as the main constituent material Co, Fe, or another ferromagnetic metal, thermal conductivity is generally high. The protective layer on this magnetic recording layer comprises diamond-like carbon (hereafter “DLC”), which has a still higher thermal conductivity. Hence after light irradiation for thermal assist, the temperature of the protective layer and of the lubricant layer deposited on the protective layer rises to approximately the same temperature as the magnetic recording layer due to the heat conveyed by successive thermal conduction from the underlayer, intermediate layer, and magnetic recording layer.
With respect to such a magnetic recording medium for thermally assisted recording, a magnetic recording medium is known which comprises a recording film having, at least, a recording film with magnetic anisotropy on a disk substrate, and comprising, on the recording film, at least a lubricant layer, with a protective layer with lower thermal conductivity than the recording film interposed therebetween. It has been disclosed that, by means of a magnetic recording medium configured in this way, thermal transmission to the lubricant layer due to the temperature increase in the magnetic recording layer is blocked, and temperature increases in the magnetic head used for recording and reproduction can be prevented (WO2005/083696 pamphlet (Embodiment 3, FIG. 3)).
In a magnetic recording medium of the prior art, when the medium is heated by light irradiation in thermally assisted recording, in addition to the magnetic recording layer in question, the temperature of the protective layer and lubricant layer also rises due to thermal conduction from the magnetic recording layer. The protective layer and lubricant layer comprise materials such as organic polymers and amorphous carbon which have relatively low thermal resistance, and so there is a high risk of performance degradation due to heating.
To provide further explanation of performance degradation due to heat in the protective layer and lubricant layer, the protective layer is formed using DLC, and the lubricant layer is formed using polymers of perfluoro polyether (hereafter “PFPE”) with a molecular weight of approximately 1000 to 10,000. By adopting this configuration, the protective layer and lubricant layer each play important roles in ensuring the long-term reliability of the magnetic recording medium; however, both materials have the property of being vulnerable to heat. As a result, when thermally assisted recording is performed using a magnetic recording medium of the prior art, the DLC protective layer undergoes film degradation and a reduction in thickness due to oxidation, and the PFPE lubricant layer undergoes characteristic degradation due to thermochemical molecular reactions as well as a reduction in thickness due to evaporation, and so there is a heightened rise of the occurrence of such problems as worsened reliability, shortened lifetimes, and increased rates of damage.